Catalyst for purification of lean-burn engine exhaust gas

ABSTRACT

A dual-phase zeolite having a transition metal-containing zeolite phase and a transition metal-containing oxide phase. The catalytic material may be an intimate mixture of a phase-layered structure of a first phase constituted preferably of a copper-containing high silica zeolite and a second phase constituted of copper-containing zirconia. 
     Methods are also disclosed for making a single-stage catalyst for removing NO x  and HC at high efficiency in an oxygen-rich automotive exhaust gas, and for treating the exhaust gas with the dual-phase catalyst above.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to the technology of engine exhaust gaspurification, and more particularly to the technology of removing NO_(x)at high efficiency in an oxygen-rich atmosphere from lean side air/fuel(A/F) ratio combustion.

2. Discussion of the Prior Art

Much of the prior art catalysts were designed to operate in a somewhatoxygen-deficient environment since engines were operated at or aboutstoichiometric combustion. Optimally, commercially available automotivecatalysts will not promote reduction of NO_(x) in an environment ofexcess oxygen; the traditional reductant, rhodium, is restricted to anarrow window of A/F if ammonia production is to be avoided.

High silica zeolites (2XM/_(n).XAl₂ O₃.YSiO₂) of the transition metalion exchange type, when loaded on an alumina carrier, have demonstrateda desirable ability to act as a molecular sieve and create active sites(a surface on which NO can compete with O₂ for reacting with areductant), thus permitting reduction of NO_(x) to take place in anoxidizing environment.

One of the earliest applications of high silica zeolites to thepurification of engine exhaust gases is disclosed in U.S. Pat. No.4,297,328, wherein a copper exchanged zeolite is deployed. The copper ismost effective as the ion exchange metal because it is active at lowertemperatures (such as present in a lean-burn engine exhaust) than othermetals known to date. Such catalyst was used to perform as a three-waycatalyst in an oxidizing environment and was found to initially possessa high absorption capacity for organic materials at high temperatureswithout preference for water.

Unfortunately, transition metal containing zeolites degrade at hightemperatures usually found in automotive exhaust systems. The catalyst,that is, the transition metal, tends to react with the alumina of thezeolite and form an aluminate which acts as a low surface area materialpreventing the transition metal from being actively catalytic andthereby reduces the catalytic activity of the entire system.

SUMMARY OF THE INVENTION

The invention is a dual-phase zeolite having a transition metalcontaining zeolite phase and a transition metal containing oxide phase.The catalytic material may be an intimate mixture or a phase-layeredstructure of a first phase constituted preferably of a copper-containinghigh silica ZSM5 zeolite and a second phase constituted ofcopper-containing zirconia.

The invention, according to another aspect of this invention, isdirected to a method of using or treating automotive exhaust gases,including the step of contacting the exhaust gases with such dual-phasezeolite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a comparison of NO_(x) conversion efficiency as a functionof propylene concentration in the gas flow treated over (i) aconventional CuZSM5 monolithic catalyst, and (ii) over a CuO/ZrO₂/CuZSM5 monolithic catalyst in conformity with the preferred embodimentof this invention.

FIG. 2 graphically shows a comparison of hydrocarbon conversions over awide range of hydrocarbon concentrations (propylene) when using aconventional CuZSM5 monolithic catalyst as compared to a CuO/ZrO₂/CuZSM5 monolithic catalyst according to an embodiment of the presentinvention.

DETAILED DESCRIPTION AND BEST MODE

The catalyst of this invention comprises a combination of two materialsor phases, a transition metal-containing oxide and a transitionmetal-containing zeolite. This combination may be in the form of anintimate mixture of the two materials or phases, or in the form of alayer of one material carrying a layer of the other (preferably a layerof transition metal-containing oxide on top of the transition metalcontaining zeolite which is deposited on a substrate). Transition metalherein is limited to the class of copper, cobalt, nickel, chromium,iron, manganese, silver, zinc, calcium, and compatible mixtures thereof;transition metal includes the elemental metal itself as well as themetal oxide thereof. The transition metal present in the two materialsof this catalyst preferably are the same, that is, if the zeolitecontains copper, then the oxide contains copper. Preferably, thistransition metal is copper; copper is particularly preferred because itis active at lower temperatures.

Generally, the phase materials can be employed in a ratio between about10-90% and 90-10% by weight (preferably they are employed in a weightratio between about 40-60% and 60-40% by weight).

Oxide

The transition metal-containing oxide can be made by various techniques,as will be apparent to one skilled in the art in view of the presentdisclosure. Preferably, the oxide is zirconia and the metal it containsis copper. Other oxides operable for the purpose of this invention canbe selected from the group consisting of lanthana, titania, silica,zirconia, and very minor proportions of lanthanum aluminate or bariumhexaluminate. Zirconia is most desired because it has a low interactionwith support metals or oxides. However, titania and silica each willreduce poisoning and thus provide some inventive benefit.

Copper-containing zirconia will be discussed herein as representative ofan embodiment of such transitional metal-containing oxide according tothe present invention. One method of making a copper-containing zirconiacomprises soaking a quantity of zirconia, in the form of a fine powder,repeatedly, if desired, in a solution of a copper compound, subsequentlydried, then calcined at an elevated temperature between 300°-600° C.,often at about 450° C. Alternatively, a zirconium nitrate solution maybe combined with a copper compound to produce a precipitate.

The copper compound should be one that is soluble or that can bedispersed in a liquid, that is, those which are soluble in an aqueoussolution or which can be solublized therein, e.g., with the aid of anacid or base. Exemplary of such copper compounds are copper salts likecopper nitrate and copper sulphate; organo-copper compounds likecarboxylates of copper, copper acetate, and copper-cupric amines;organo-complexes of copper like diamine copper acetate; tetraaminecopper sulphate, and copper acetylacetonate. Soluble compounds,exemplary of other transition metal compounds, include cobalt acetate,nickel acetate, ferric chloride, chromic nitrate, and manganese acetate.

The saturated zirconia is then dried and calcined in air, the coppercompound decomposing to form copper oxide. If, on the other hand,calcining is carried out in a reducing atmosphere, the copper compoundmay be reduced to elemental copper. Preferably, copper is present in anamount between about 0.1-20% by weight in the copper-containing oxide.When the first phase of the catalyst is placed in use, oxygen present inthe exhaust gas will oxidize the copper to copper oxide.

Zeolite

In addition to the transition metal-containing oxide, the catalystcontains a transition metal-containing zeolite. The zeolite is desirablya high silica zeolite having an SiO₂ /Al₂ O₃ molar ratio which exceedsabout 10, preferably up to about 60 (see U.S. Pat. No. 4,297,328, whichis expressly incorporated herein by reference for teaching of otherzeolites or class of zeolites that may be used herein).

The transition metal, such as copper, is provided into the zeolite byion exchange. Again, the transition metal may be selected from the groupconsisting of Cu, Co, Ni, Cr, Fe, Mn, Ag, Zn, Ca, and compatiblemixtures thereof. Generally, a sodium, hydrogen, or ammonium zeolite iscontacted by an aqueous solution of another cation, in this case anaqueous solution of a soluble copper compound such as copper acetate,wherein replacement of the sodium, hydrogen, or ammonium ion by copperion takes place. It is advantageous to provide as much transition metalion in the zeolite as possible since the amount of transition metalpresent in the zeolite is directly related to the catalytic activity ofthe first phase catalyst. Preferably, this is at least 3% up to amaximum determined by the SiO₂ /Al₂ O₃ ratio. After replacing thesodium, hydrogen, or ammonium ion with the metal ion, the zeolite isgenerally washed to remove excess surface transition metal compound. Itis not necessary to do so, however. The catalyst may further bemanufactured by grinding each of the copper-containing oxide and thecopper-containing zeolite to a fine powder, mixing them together,forming a slurry of them, and then applying the slurry to a substratesuch metal or ceramic honeycomb. While it is preferable to make thecatalyst in this way, it may be made by layering one material onto theother.

A series of catalyst examples were prepared to corroborate the scope ofthe invention herein.

EXAMPLE

(a) A copper acetate monohydrate (7.8 g) is dissolved in 150 ml ofacetic acid/water mixture (50:50). Zirconia, 50 g, is placed in a beakerand is impregnated with the copper acetate solution by incipient wetnesstechnique. Three impregnations are necessary to finish all of thesolution. The impregnated material is dried at 120° C. betweenimpregnations and after the final impregnation. The impregnated materialwas then calcined at 600° C. for four hours.

(b) A high silica zeolite was ion-exchanged with copper; obtained from acommercial source as Cu-ZSM-5. The material contained 3% by weightion-exchanged copper and was in a powder form suitable for direct use.

(c) A mixture was prepared from equal amounts of materials (a) and (b)above and ball-milled into a slurry in distilled water. The slurry wasapplied to a cordierite monolith carrier in several steps to obtain a33% loading of the material of the aggregate catalyst sample weight. Thefinal drying was carried out at 120° C. for three hours and calcinationis carried out at 600° C. for four hours.

Comparisons were undertaken to establish the efficiency of the catalystto convert hydrocarbons and nitric oxide in an automotive exhaust-typegas. The simulated exhaust gas treatment tests were then used to comparethe hydrocarbon conversion efficiency of this example with reference toa conventional copper exchanged high silica zeolite (Cu-ZSM-5). Theconditions under which the test was performed included an inlet gasblend of 3.8% O₂, 500 ppm NO, 20 ppm SO₂, 10% H₂ O, 12% CO₂, and thebalance nitrogen. The catalyst substrate was tested at a space velocityof about 50,000 hr⁻¹ and at a gas temperature of about 900° F. As shownin FIG. 1, the variation of nitric oxide conversion efficiency with thevariable content of propylene in the synthesized exhaust gas isdisplayed. The much greater nitric oxide conversion efficiency of thisinvention is evident from such Figure. At all data points, the NO_(x)conversion is roughly 2:1 better when the catalyst contains ZrO₂ withCuO. This proves that more copper is available as catalyst for reductionbecause it is prohibited from combining with the Al₂ O₃ by forming itsown oxide.

Similarly, a comparison of hydrocarbon conversion efficiency wasdetermined as shown in FIG. 2. Again, the catalyst of this inventionprovided a superior hydrocarbon conversion efficiency compared to thatof a conventional copper ion-exchanged zeolite.

As shown in Table I, variations of the oxide as well as the zeolite inthe second phase catalyst were made to determine the affect, if any, onthe conversion efficiency of NO_(x) in an automotive exhaust gas. Thetemperature of the treated exhaust gas was about 425° C., space velocityof 50 K hr⁻¹.

Examples 2 through 4 were prepared in accordance with the procedure usedto prepare Example 1, except that in each case a different transitionmetal was used for formulating the zirconia oxide and the ion exchangeof the zeolite. Example 3, with Ag, worked quite well at the 425° C.temperature. Substituting other refractory oxides for zirconia (Examples5-8) showed little change in the NO_(x) conversion efficiency.

With respect to Examples 9-10, the base zeolite itself was varied aslisted while maintaining the copper modified zirconium oxide and copperexchange in the zeolite as a constant. The conversion efficiency forthese examples showed that the total Cu content was progressivelylimited which reduced NO_(x) conversion efficiency somewhat. Examples11-13 varied the ratio between the first and second phases of suchsingle-stage catalyst.

                                      TABLE I                                     __________________________________________________________________________    Oxide        Zeolite                                                                              Oxide/                                                                            SiO.sub.2 /                                                                       Conversion Efficiency                             Example                                                                            TM/     TM/    Zeolite                                                                           Al.sub.2 O.sub.3                                                                  NO.sub.x                                                                          NC CO                                         __________________________________________________________________________    1    CuO/ZrO.sub.2                                                                         Cu/ZSM-5                                                                             1/1 40  48  75 3                                          2    CoO/ZrO.sub.2                                                                         Co/ZSM-5                                                                             1/1 40  50  70 5                                          3    Ag.sub.2 O/ZrO.sub.2                                                                  Ag/ZSM-5                                                                             1/1 40  55  65 10                                         4    ZnO/ZrO.sub.2                                                                         Zn/ZSM-5                                                                             1/1 40  25  30 0                                          5    CuO/Ba,Al.sub.2 O.sub.3                                                               Cu/ZSM-5                                                                             1/1 40  45  74 3                                          6    CuO/La.sub.2 O.sub.3                                                                  Cu/ZSM-5                                                                             1/1 40  48  74 3                                          7    CuO/TiO.sub.2                                                                         Cu/ZSM-5                                                                             1/1 40  47  75 3                                          8    CuO/SiO.sub.2                                                                         Cu/ZSM-5                                                                             1/1 40  47  74 3                                          9    CuO/ZrO.sub.2                                                                         Cu/mordenite                                                                         1/1 60  40  72 5                                          10   CuO/ZrO.sub.2                                                                         Cu/ferrierite                                                                        1/1 10  30  69 4                                          11   CuO/ZrO.sub.2                                                                         Cu/ZSM-5                                                                             1/2 40  39  72 3                                          12   CuO/ZrO.sub.2                                                                         Cu/ZSM-5                                                                             1/3 40  30  70 3                                          13   CuO/ZrO.sub.2                                                                         Cu/ZSM-5                                                                             1/4 40  25  65 3                                          __________________________________________________________________________

We claim:
 1. A catalyst for purification of lean-burn engine exhaustgas, comprising:a dual-phase, high silica zeolite supported on a highsurface area substrate, the first phase consisting of a catalytictransition metal containing oxide and the second phase consisting of acatalytic transition metal containing high silica zeolite.
 2. Thecatalyst as in claim 1, in which said substrate is an alumina carrier.3. The catalyst as in claim 1, in which said transition metal isselected from the group consisting of Cu, Co, Ni, Cr, Fe, Mn, Ag, Zn,Ca, and compatible mixtures thereof.
 4. The catalyst as in claim 1, inwhich said zeolite has a SiO₂ /Al₂ O₃ molar ratio greater than
 10. 5.The catalyst as in claim 4, in which said zeolite is ultrastable, of theform 2XM/_(n).XAl₂ O₃.YSiO₂, where M is the cation and n is the valance.6. The catalyst as in claim 1, in which said zeolite, regardless of themetal replacing the cation, retains a spatial arrangement of aluminum,silicon, and oxygen that forms a basic crystal lattice that remainsessentially unchanged.
 7. The catalyst as in claim 1, in which saidoxide is selected from the group consisting of zirconia, lanthana,titania, silica, zirconium aluminate, and barium hexaluminate.
 8. Thecatalyst as in claim 1, in which the ratio of said phases is in therange of 90/10-10/90.
 9. The catalyst as in claim 1, in which said oxideis zirconia present in an amount of 0.1-20 weight percent of saidcatalyst.